We propose to locate the tissues responsive to relaxin in the rat. We will employ a sensitive and precise radioimmunoassay for relaxin to determine factors which influence the secretion of relaxin in pregnant pigs and rats. BIBLIOGRAPHIC REFERENCES: O.D. Sherwood, C.C. Chang, G.W. Bevier, and P.J. Dziuk. Radioimmunoassay of Plasma Relaxin Levels Throughout Pregnancy and at Parturition in the Pig. Endocrinology 97:834, 1975. O.D. Sherwood, C.C. Chang, G.W. Bevier, J.R. Diehl, and P.J. Dziuk. Relaxin Concentrations in Pig Plasma Following the Administration of Prostaglandin F2 alpha During Late Pregnancy. Endocrinology 98:875, 1976.